2 §15.1 The First Law of Thermodynamics The first law of thermodynamics says the change in internal energy of a system is equal to the heat flow into the system plus the work done on the system. W Q U + = ∆

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4 Question 8 • A gas expands, doing 100 J of work. How much heat must be simultaneously added to this system for its internal energy to increase by 200 J? • A) 100J • B) 200J • C) 300J • D) 400J

5 §15.2 Thermodynamic Processes A state variable describes the state of a system at time t, but it does not reveal how the system was put into that state. Examples of state variables: pressure, temperature, volume, number of moles, and internal energy.

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6 A PV diagram can be used to represent the state changes of a system, provided the system is always near equilibrium. The area under a PV curve gives the magnitude of the work done on a system. W>0 for compression and W<0 for expansion.

7 The work done on a system depends on the path taken in the PV diagram. The work done on a system during a closed cycle can be nonzero. To go from the state (V i , P i ) by the path (a) to the state (V f , P f ) requires a different amount of work then by path (b). To return to the initial point (1) requires the work to be nonzero.

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10 §15.3 Thermodynamic Processes for an Ideal Gas No work is done on a system when its volume remains constant (isochoric process). For an ideal gas (provided the number of moles remains constant), the change in internal energy is . T nC U Q V ∆ = ∆ =